1. Field of the Invention
The present invention relates to a method and apparatus for molding an inflation film. More specifically, the present invention relates to a method and apparatus for molding an inflation film for producing a thermoplastic resin film having excellent transparency (i.e., haze, gloss and clarity) and high strength, at a high productivity, which is capable of molding, at a high molding rate, synthetic resins which have a small melt tension and hence have been believed difficult to mold into a long neck type bubble (i.e., a bubble shape of the type in which a bubble expansion position is considerably spaced apart from a die), such as ethylene copolymer resins such as linear low density polyethylene (hereinafter referred to as "LLDPE"), low density polyethylene (hereinafter referred to as "LDPE"), or high density polyethylene (hereinafter referred to as "HDPE"), ethylene-vinyl acetate copolymer resins and ethylene-acrylate-based monomer copolymer resins, polypropylene resins, polyamide resins, polyester resins, according to a long neck type inflation film molding method, whereby films having a high strength are provided.
2. Description of the Related Art
Many proposals have been made about the production methods of inflation films by an air cooling method. Among them, the shapes of resin bubbles for molding the film can be classified broadly into four or five types (See, for example, FIGS. 1 and 2, and FIGS. 9 and 10).
Factors which determine the shape of the melt resin bubble include, for example, a cooling capacity, a film take-up speed and a melt resin temperature. In the case of linear polyethylene having a high melt tension such as ultra-high molecular weight high density polyethylene (hereinafter referred to as "HMWHDPE"), film molding by a so-called "longneck" type bubble has been widely employed, and the resulting film is supplied in large quantities as a balance film having a high strength in the field of, for example, shopping bags.
According to this method, however, since the melt bubble is gradually cooled, a transparent film cannot be obtained.
On the other hand, LLDPE has an extremely low melt tension and the fluidizing characteristics thereof are remarkably different from those of HMWHDPE. Therefore, the stability of the bubble is low and it is difficult to shape the bubble into, for example, a long neck type shape. Generally, the bubble is molded in a bubble shape of the type shown in FIG. 9 or 10 (which is called a "low frost line type") or by a T-die method. This is also true in the case of other thermoplastic resins having a low melt tension.
According to this low frost line type production method of the inflation film, the melt bubble is quenched and hence, a transparent film can be obtained. However, this method involves the problems in that the stability of the bubble becomes low in the case of a high speed production, and variance of the thickness and width of the film is likely to occur due to swaying of the bubble. Furthermore, orientation becomes larger in a take-up direction when the film is produced at a high speed. Therefore, the strength of the film in a longitudinal direction (i.e., machine direction) increases, whereas the strength in a direction perpendicular thereto (i.e., transverse direction) is likely to be greatly decreased, and the balance of the strength is lost. Accordingly, the film is likely to be broken in the longitudinal direction and the take-up speed is difficult to be increased. Thus, the productivity is limited.
As mentioned above, in order to mold a film from a synthetic resin having a low melt tension by an inflation method, it has been heretobefore necessary to either form a low frost line type bubble shape, which can be stably produced, although the take-up speed is low, or to employ a T-die method.